General synonyms:
* Playful
* Sprightly
* Lively
* Energetic
* Vigorous
* Spirited
* Buoyant
* Animated
* Full of life
More specific synonyms:
* Naughty (for a child or animal)
* Impish (for a mischievous person)
* Fidgety (for someone restless or easily distracted)
* Giddy (for someone feeling lightheaded or overly happy)
* Nimble (for someone quick and agile)
The best synonym to use will depend on the specific situation and the connotation you want to convey.